The present invention relates to an apparatus for charging a substrate comprising a dielectric substrate, a support member having a first end and a second end between which said member extends substantially parallel to the substrate, a row of electrodes distributed over the support member between the first and second ends, said electrodes extending from the support member in the direction of the substrate, the electrodes having a free end for spraying a charge on the substrate, each free end having a predetermined, mainly fixed distance from the substrate. The present invention also relates to an image forming apparatus provided with an apparatus of this kind.
An apparatus of this kind is known from Japanese Laid-Open specification JP 03164467, in which a corona unit is described which can be used in an image forming apparatus, for example a printer, copying machine or fax. Apparatus of this kind frequently makes use of an image forming process wherein a dielectric substrate, usually a photoconductor, is uniformly charged in a first step by use of a corona unit. For this purpose, a pin corona is used in JP 03164467. In a corona of this kind, a large number of individual electrodes extend from the support member in the direction of the substrate which is to be charged. In this case the support member is an elongated electrically conductive member of a length substantially equal to the width of the substrate for charging. In this way the substrate can be charged over its full width by passing it beneath the corona unit. At their free ends the electrodes are provided with a sharp point with a very small radius. By the application of a high voltage between the electrodes and the substrate, a charge will be sprayed from the point of the electrode in the direction of the substrate. Since the substrate is dielectric, charge can thus be built up on the surface of the substrate. In a following step, a charge image is then created on the substrate by discharging the substrate imagewise. This charge image can then be converted to a visible image by developing it with toner. To obtain good image quality, it is important that a uniform charge is applied to the surface of the substrate during the initial charging. Frequently, a non-uniform charge of the substrate is obtained due to all kinds of effect. For example, at the ends of the support member there are edge effects which are perceptible. Since the outermost electrodes have no more neighboring electrodes, there will often be less charge sprayed on to the substrate in the vicinity of these ends per unit area. JP 03164467 does not give any solution for this problem. By bending the support member of the corona unit in the vicinity of the respective ends in a plane parallel to the substrate, the effective electrode density is greater at these ends. In this way the lower charge in the vicinity of the ends can be compensated.
However, the apparatus known from JP 03164467 has one significant disadvantage. Since the support member has a bend, it is practically impossible to clamp this element in a corona unit while retaining the required bend. Clamping of this kind is necessary in order to obtain a reliable location of the support member in the corona charging apparatus. In the absence of a reliable location of this kind, the charging level of the substrate cannot be obtained at a predetermined and uniform value, which has an adverse effect on the final image quality.
The object of the present invention is to obtain an apparatus for charging a substrate whereby a predetermined uniform or substantially uniform charging of the substrate can be obtained. Toward this end, an apparatus has been developed, wherein the said distance in the case of a first electrode differs substantially from said distance in the case of a second electrode.
It has been found that charging the substrate at the location of an individual electrode can be influenced by varying the distance between the free end of the electrode and the substrate. As soon as a change of this distance results in a perceptible change in the charging level of the substrate (approximately one volt is already perceptible), this may be referred to as a substantial change. A smaller distance often results in a stronger charging of the substrate, while a larger distance results in a weaker charge. If the charge is locally too weak, for example at the edge of the substrate, the charge can be brought to an adequate level by precisely reducing, at that location, the distance between the free end of one or more electrodes and the substrate. A change in the distance equal, for example, to one-tenth of a millimeter will result in a perceptible change of the charging level of the substrate at the corresponding electrode. If the charge is locally too strong for any reason whatsoever, then the distance of the free end of one or more electrodes corresponding to that location could be increased. It is thus possible in an effective manner to obtain a corona unit which has no bend but nevertheless results in an adequately uniform charging of the substrate. Since the support member can be made completely straight, it can be fixed in a known manner in the corona unit so that a reliable location with respect to the substrate can be obtained. Therefore, by using an apparatus according to the present invention, a predetermined uniform or almost uniform charging of the substrate can be obtained.
Charging apparatus in which pin coronas are used is also known from U.S. Pat. Nos. 3,655,966 and 5,083,959, in which the free ends of the electrodes do not extend through a straight line. Nevertheless, it is known from both patent specifications that each of the free ends must have the same distance from the substrate for charging, in order to obtain a uniform charge. The configuration of the free ends is used as an adaptation to a bend of the associated substrates. As a result, these known devices still have the problem of a non-uniform charge. This means that these devices are further away from the present invention than the above-described apparatus.
In one embodiment of the present invention, said distance for electrodes in the vicinity of the first and second ends of the support member is less than the distance for electrodes in the vicinity of the middle of the support member. It has been found that a weaker charge occurs particularly at the edges of the substrate, in many corona units. The apparatus according to this embodiment solves this problem by placing the free ends of one or more electrodes in the vicinity of these edges more densely in the neighborhood of the substrate, so that the charging level is locally influenced positively. By selecting a suitable distance, it is possible in principle completely to compensate for the difference in charging level so that a uniform charge is obtained.
In another embodiment of the invention, the electrodes in the row are substantially congruent and the support member has a side for carrying the electrodes, said side being profiled. In this embodiment, the electrodes themselves are substantially identical and of the same shape. This has the advantage that the spraying behavior of the electrodes is also substantially equal, and this has a favorable influence on image quality. A difference in the distance of the free end of an electrode from the substrate is obtained in this embodiment by locally displacing the support member, at least that side of said member which carries the electrodes. As a result, this side acquires a profile. A profile of this kind can assume many shapes. If only one end of an electrode is to be displaced, said profile can, for example, be a step function. In a practical embodiment, however, it will more frequently be the case that a number of adjoining free ends must be displaced. This can be done, for example, by giving the associated side of the support member a gradually extending profile.
In another embodiment, said side of the support member is so profiled that said side forms part of a polygon, each side of the polygon carrying at least one electrode. In this embodiment, the profile extends gradually, via a number of straight support elements merging into one another and which in each case form a different angle with the substrate. It has been found that a support member according to this further embodiment is simple to produce because the electrodes themselves can remain in the same form.
In a further embodiment, the sides of the polygon which form an angle with the substrate other than mainly 180xc2x0 carry a maximum of five electrodes. It has been found that with an apparatus according to this embodiment a predetermined uniform of the substrate can be obtained in simple manner. Since the number of electrodes for each support element is restricted to a maximum of five, the charging level can be accurately controlled.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.